Regenerative thermal oxidizers are conventionally used for destroying volatile organic compounds (VOCs) emissions from industrial, manufacturing and power plants. Such oxidizers typically require high oxidation temperatures in order to achieve high VOC destruction and utilize high heat recovery efficiency. To more efficiently attain these characteristics, the xe2x80x9cdirtyxe2x80x9d process gas which is to be treated is preheated before oxidation. A heat exchanger column or bed is typically provided to preheat these gases. The column is usually packed with a heat exchange material having good thermal and mechanical stability and high thermal mass. In operation, the process gas is fed through a previously heated heat exchanger column, which, in turn, heats the process gas to a temperature approaching or attaining its VOC oxidation temperature. This pre-heated process gas is then directed into a combustion chamber where any incomplete VOC oxidation is usually completed. The treated xe2x80x9ccleanxe2x80x9d gas is then directed out of the combustion chamber and back through the heat exchanger column, or through a second heat exchange column. As the hot oxidized gas is fed through the second column, the gas transfers its heat to the heat exchange media in that column, cooling the gas and pre-heating the heat exchange media so that another batch of process gas may be preheated prior to the oxidation treatment. Usually, a regenerative thermal oxidizer has at least two heat exchanger columns which alternately receive process and treated gases. This process is continuously carried out, allowing even large volumes of process gas to be efficiently treated.
The performance of a regenerative oxidizer may be optimized by increasing VOC destruction efficiency and by reducing operating and capital costs. The art of increasing VOC destruction efficiency has been addressed in the literature using, for example, means such as improved oxidation systems and purge systems. Operating costs can be reduced by increasing the heat recovery efficiency, and by reducing the pressure drop across the oxidizer. Operating and capital costs may be reduced by properly designing the oxidizer and by selecting appropriate heat transfer packing materials.
A typical conventional regenerative thermal oxidizer includes a burner. The burner is used during start-up to bring the apparatus up to operating temperature. Once operating temperature is achieved, the burner output is lowered, and if sufficient VOC""s are present in the process gas, the burner is preferably placed in pilot mode with the hope that the system will remain at the desired temperature due to the oxidation of the VOC""s in the process gas. Any additional use of the burner during operation is not cost effective. Indeed, even in pilot mode, the burner is consuming fuel and is introducing ambient combustion air into the apparatus which must be heated to the operating temperature, thereby further reducing overall efficiency. Typically, nozzle mix burners require 25% of full flow combustion air during pilot operation.
In an effort to minimize use of the burner during operation of the oxidizer, it is known to apply auxiliary heat to a regenerative thermal oxidizer using fuel injection. Such auxiliary heat may be necessary, for example, when the concentration of VOC""s in the process gas decreases at any given time, thereby not allowing the oxidizer to sustain the desired operating temperature due to the relatively low concentration of VOC""s to oxidize. These systems typically employ the method of introducing gaseous fuel at or near the inlet point of the oxidizer. The injection location is typically just upstream or downstream of the inlet flow control valves or rotary distributor, as the case may be. The intent is to mix the fuel with the process gas prior to the gas stream flowing into the inlet heat exchange bed of the oxidizer. As the process gas flows through the inlet bed, it picks up the heat from the surrounding heat exchange media, and eventually passes the ignition temperature of the fuel. When the fuel-laden gas is heated sufficiently, the fuel oxidizes (burns), giving off heat to the process gas. The amount of fuel gas injected is controlled to maintain proper incineration temperature in the oxidizer. One example of such as system is described in U.S. Pat. No. 4,267,152. Temperature is sensed in the oxidizer combustion chamber, and when that temperature is at a predetermined level, fuel gas is supplied to the oxidizer combustion chamber by mixing it with the incoming effluent before application to the combustion chamber. Temperature uniformity in the combustion chamber and fuel cost savings are objectives of such a system.
In practice, however, it has been difficult to control the location in the heat exchange column where the injected fuel autoignites. Specifically, if one assumes that the desired oxidation temperature is 1600xc2x0 F., the injected fuel gas ignites and creates that temperature in the heat exchange column at a location well upstream of the combustion chamber, perhaps at the midpoint of the bed. This results in the remaining portion of the heat exchange column through which the effluent flows becoming heat soaked and thus redundant, since no further heat exchange will take place in that portion of the bed. In addition, capital equipment costs are increased, as two gas trains are required, one to feed fuel to the burner in the combustion chamber and one for the injected fuel gas. Safety concerns, such as bed plugging and valve sticking, are also significant, and requirements to address these and other safety issues are costly. From an operational standpoint, the fuel gas injection must be stopped during each valve cycle to avoid dangerous gas build-up.
It is therefore an object of the present invention to provide a heat source such as for a regenerative thermal oxidizer which creates a stable flame and is efficient.
It is a further object of the present invention to provide a burner for a combustion chamber that does not introduce unnecessary ambient combustion air into the apparatus.
It is another object of the present invention to provide a burner which allows for the introduction of a fuel gas, via the burner, directly into the combustion zone of a regenerative thermal oxidizer.
It is yet another object of the present invention to provide a burner that self-regulates the amount of combustion air necessary to form and maintain a flame.
It is still another object of the present invention to provide a burner that does not require a combustion blower to supply combustion air to the burner for combustion of a fuel gas.
It is a further object of the present invention to eliminate the known potential safety hazards associated with injecting gas upstream of the combustion chamber.
It is a still further object of the invention to eliminate unburned fuel gas to be emitted during mode changes.
The problems of the prior art have been overcome by the present invention, which provides a burner that utilizes venturi action for induction of combustion air. In a preferred application, the burner of the present invention is used in a regenerative thermal oxidizer having one or more heat exchange beds associated with a combustion chamber or zone. The burner is preferably located in the combustion zone, such as at a location which is near or at the midpoint between the inlet and outlet heat exchange beds. The burner allows for the control of the location of the heat release.
In order to avoid the resulting high flame temperatures which would create high NOx formation and uneven heat distribution, fuel gas is injected directly into a burner designed to exhibit venturi action. The action of the venturi draws in surrounding (hot) process gas present around the nozzle to supply the necessary combustion air, provide forward moment to the burning gases to distribute their heat, and control the location of the heat release. The burner can thus be used to accomplish fuel gas injection directly into the combustion chamber of an oxidizer, and does not require a separate gas train or suffer from the other various drawbacks typical of conventional fuel gas injection systems. A stable flame is generated, and efficient heat-up accomplished. No extra combustion air is necessary.